Some of the disease states that pose the greatest danger to vision are thought to involve abnormalities in the oxygen supply to the retina or the oxygen metabolism. These include diseases such as diabetic retinopathy, which is one of the most common causes of blindness in the world, retinopathy of prematurity, retinal vein and artery occlusions. It has also been suggested that ocular blood flow is compromised in glaucoma, including normal tension glaucoma, and that the loss of optic nerve fibers in glaucoma patients may be due to ischemia. Age related macular degeneration may also involve ischemia and hypoxia.
The principle of spectrophotometric hemoglobin oximetry is old and the method has been developed into clinically useful instrumentation in several specialties of medicine. The finger or earlobe oximeter, commonly used in anesthesiology and intensive care is an example of the successful application of oximetry in medicine.
Information on retinal and optic nerve oxygenation in health and disease is mostly derived from animal research. Non-invasive measurement of oxygenation in the human retina and optic nerve has proved difficult but considerable progress has been made, particularly in recent years as digital technology has evolved. The potential use of retinal oximetry covers a range of areas, including assessment of oxygen metabolism in disease, the efficacy of treatment of these diseases in restoring or improving the metabolic conditions, either by laser treatment, surgery, lowering of intraocular pressure or by medication. Retinal oximetry may also be of use in elucidating further the physiological processes involved in disease states, such as in glaucoma and diabetic retinopathy.
In the 1990s, an important version of retinal vessel oximetry was developed at the University of Virginia, by James Beach and James Tiedeman. Their version was based on a method proposed by Delori with additional improvements. The main advantage of the Beach and Tiedeman method is the ability to obtain simultaneously two or more images of light reflectance at different wavelengths (called multi-spectral images) from the same fundus using a fundus camera. In this manner it is possible to record reflectance at both oxygen-sensitive and insensitive wavelengths from exactly the same area on the fundus, and at precisely the same time. This allows for precise quantification of the effects of oxygen binding on the light absorption spectra of hemoglobin. Oximetric measurements of this kind are achieved by a system whose main component consists of a modified fundus camera, although the internal xenon flash of the camera is still used as a light source, unmodified. Other components of the system are a beam splitter, a gray scale digital camera, possessing high quality linear performance, and a computer. The digital camera replaces the image acquisition mechanism of the fundus camera. Flashes from the fundus camera are synchronized with recordings by the digital camera electronically.
More sophisticated retinal oxymetry is disclosed in detail in US20060276698A1 patent application. The system is capable of automatically evaluating oxygen saturation of the optic nerve and retina. The system is comprised of an image capturing system which further comprises fundus camera, a beam splitter, a digital image capturing device, a computer system, image processing software performing in real-time the steps of registering a set of multi-spectral images. This is accomplished by binarizing multi-spectral image, finding all the border regions of each image by finding the region including the straight line that passes the highest number of points in the region. The orientation of the borders is used to evaluate the orientation of each spectral image, equalize the orientation of each spectral image by rotating the spectral image, edge detect each spectral image, and estimate the translation between the spectral images based on the edges of adjacent images. The images are then transformed to a stack of registered images, were blood vessels are located in each of the spectral images. The oxygen saturation level is finally evaluated, after sophisticated image processing, and the results are presented.
WO02080759 discloses retinal camera for examining an eye, the camera including a light source having first and second light sources for emitting first and second wavelength bands. The first and second light sources are arranged to alternately produce light onto the retina such that the absorptivity of light of the first wavelength band by oxygenated blood is greater than the absorptivity of light of the second wavelength band, and the absorptivity of light of the first wavelength band by the oxygenated blood is less than the absorptivity of light of the second wavelength band. Light is selectively focused from the first and second sources by an optical arrangement and imaging devices to produce respective images of a portion of the retina illuminated with the respective wavelength bands. The images obtained by the imaging device are processed by the imaging device and processor to determine a retinal metabolic image based on haemoglobin oxygenation. In an embodiment of the invention light is scanned across the retina.
This makes it possible for clinicians to evaluate the oxygen supply to the retina or the oxygen metabolism and the oxygen saturation of the retina and optic nerve. However, it would be extremely valuable to be able to compare successive measurements of the same optic nerve and retina.